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According to Wikipedia it looks like the (or at least a) proper translation of open source is código abierto. I'm pretty sure I've also seen just the phrase open source used, without translation. Likewise, I've seen free software used without translation (in fact the Wikipedia page linked even does that).

At least in Spanish the distinction between free (gratis) software and free (libre) software isn't as confusing as in English!

The great rms (Richard Stallman) uses "el software libre" in his Spanish talks, though I can't remember how he references merely "open source". I suppose like many terms it should be OK to just say "open source". At least there's a difference between "software gratis" and "software libre" :)